Lability of nanoparticulate metal complexes in electrochemical speciation analysis

Lability concepts are elaborated for metal complexes with soft (3D) and hard (2D) aqueous nanoparticles. In the presence of a non-equilibrium sensor, e.g. a voltammetric electrode, the notion of lability for nanoparticulate metal complexes, M-NP, reflects the ability of the M-NP to maintain equilibrium with the reduced concentration of the electroactive free M²⁺ in its diffusion layer. Since the metal ion binding sites are confined to the NP body, the conventional reaction layer in the form of a layer adjacent to the electrode surface is immaterial. Instead an intraparticulate reaction zone may develop at the particle/medium interface. Thus the chemodynamic features of M-NP complexes should be fundamentally different from those of molecular systems in which the reaction layer is a property of the homogeneous solution (μ?=?(D _M/k _a ^′)^1/2). For molecular complexes, the characteristic timescale of the electrochemical technique is crucial in the lability towards the electrode surface. In contrast, for nanoparticulate complexes it is the dynamics of the exchange of the electroactive metal ion with the surrounding medium that governs the effective lability towards the electrode surface.     

Theoretical aspects of 2D electrochemical phase formation

Journal of Solid State Electrochemistry - Two-dimensional electrochemical phase formation as a result of nucleation, growth, and overlap of disk-shaped nuclei of a new phase on the electrode...     

Contact probe electrochemical characterization and metal speciation of silver LLDPE nanocomposite films

A contact probe methodology, based on the voltammetry of immobilized microparticle approach, is used for characterizing silver species present in linear low density polyethylene (LLDPE) films with different Ag(I)/Ag(0) ratios and silver nanoparticle features usable as food contact polymers. The films displayed characteristic voltammetric features in contact with aqueous acetate buffer, in particular signals for the stripping oxidation of nanoparticulate Ag systems. Significant differences between the studied films were also observed by means of electrochemical impedance spectroscopy and detected at the nanoscopic scale using electrochemical scanning microscopy. Differences in optical and thermal properties of the studied films are associated with the presence of silver nanoparticles. The silver oxidation state as well as nanoparticle size also had influence on the oxidative resistance of the LLDPE films; indeed, films containing cationic silver showed the lowest oxidation induction time value.     

Iron detection and speciation in natural waters by electrochemical techniques: A critical review

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Biosampling for metal speciation

Summary Human erythrocytes (ery's) under physiological conditions are capable of a selective uptake of dissolved chromates beside Cr(III) from aqueous solutions. For sampling the mechanical stability of the red blood cells is increased by immobilization in Ca-alginate beads without loss of the biochemical activity against Cr(VI). Radiotracer studies were performed in order to determine the influence of temperature, pH, and concentration and solubility of chromates on the accumulation, which follows a Michaelis-Menten kinetics. After sampling the ery's are separated from the gel and introduced into a multiple-step clean-up procedure preceding a GFAAS measurement of chromium. The direct determination of chromium in erythrocytes (and comparable matrices) by GFAAS is supported by the use of oxygen as alternate gas during the thermal decomposition of the samples, leading to advantages in routine analysis. The statistical parameters of this method are critically reviewed. The combination of biosampling and GFAAS represents a new way for metal speciation, especially in cases where decomposition of the analyte during standard sampling procedures is likely; this is demonstrated for Cr(III)/Cr(VI) as an example in view of immission measurements of Cr(VI) in airborne particulates.Dedicated to Prof. Dr. G. Tölg on the occasion of his 60th birthday     

Generation and detection of single metal nanoparticles using scanning electrochemical microscopy techniques

Different pathways towards the generation and detection of a single metal nanoparticle (MNP) on a conductive carbon support for testing as an electrocatalyst are described. Various approaches were investigated including interparticle distance enhancement, electrochemical and mechanical tip-substrate MNP transfer onto macroscopic surfaces, scanning electrochemical microscopy (SECM)-controlled electrodeposition, and the use of selective binding monolayers on carbon fiber electrodes (CFEs) for solution-phase-selective adsorption. A novel SECM technique for electrodepositing MNPs on CFE tips immersed 100-200 nm below the electrolyte level was developed and used to generate single Pt and Ni nanoparticles. Following their generation, we demonstrate electrocatalytic detection of Fe3+ on individual Pt particles with the CFE in a Fe3+/H2SO4 solution. We also describe an approach of attaching MNPs to CFEs by controlling the composition of monolayers bonded to the CFE. By employing a monolayer with a low ratio of binding (e.g., 4-aminopyridine) to nonbinding molecules (e.g., aniline) and controlling the position of the CFE in a colloidal Pt solution with a SECM, we attached a single 15 nm radius Pt nanoparticle to the CFE. Such chemisorbed Pt particles exhibited a stronger adhesion on surface-modified CFEs and better mechanical stability during proton reduction than MNPs electrodeposited directly on the CFE.     

Preparation of metal Niacin complexes and characterization using spectroscopic and electrochemical techniques

Metal complexes of Niacin (3-pyridin carboxylic acid) were prepared in aqueous medium and characterized by different physico-chemical methods. On the basis of elemental analysis the empirical formula of the complexes have been proposed as [Fe(C₆H₄NO₂)]Cl₂, [Co(C₆H₄NO₂)]Cl, [Zn(C₆H₄NO₂)]Cl, [Cd(C₆H₄NO₂)]Cl and [Hg(C₆H₄NO₂)]Cl. IR spectral data indicate that the metal-ligand bonding occurs through nitrogen atom of aromatic ring and oxygen atom of COO⁻-group. UV-visible spectra show that Fe(III) and Co(II) complexes show d-d electronic transition in addition to π → π*, n → π* and n → σ* transitions. The Fe(II) and Co(II) complexes are paramagnetic. QSTG analysis data strongly support the absence of water molecules in the complexes, and the weight of the residue corresponds to the respective metal oxides. Cyclic voltammetric studies suggest that the redox properties of Zn(II), Cd(II) and Hg(II) in their complexes are modified compared to the uncoordinated metal ion. The CV data also indicate that the charge transfer processes are not reversible.     

Theoretical and experimental insights into the electrochemical heavy metal ion sensing with nonconductive nanomaterials

Nonconductive nanomaterial is a type of modifiers widely used in electrochemical heavy metal ion detection. Despite a large number of studies devoted to the electrochemical stripping behaviors of modifiers, a clear picture regarding the structure–performance relationship is still lacking. Recently, benefiting from the development of fine-structure characterization techniques and density functional theory calculations, the atomic details on how the surface interaction between heavy metal ions and the modifiers leads to its high sensitivity have attracted much attention. This short review discusses the development and challenge of nanomaterial-based stripping behaviors in the determination of heavy metal ions and highlights the structure–performance relationship at the atomic level.     

Use of electrochemical techniques for the conservation of metal artefacts: a review

Electrochemical techniques like mechanical and chemical ones should be among the panoply of techniques conservators normally use when they conserve metal artefacts. Often though, they are discarded because they are considered as too complicated and dangerous. As a consequence, not much development in the use of these techniques in conservation was observed before the 1990s when their application to marine artefacts once again drew the attention of conservation professionals. More recently, the latter have recognised the importance of these techniques in the understanding of corrosion processes as well as their monitoring and in the solving of specific conservation issues. Furthermore, instruments that were previously only used by corrosion scientists are today entering the conservation field. Portable tools have even been designed so that treatments can be carried out in situ. The current trend is to cluster electrochemical and analytical techniques in parallel in order to fully understand the behaviour of metal artefacts when conserved.     

Advances in arsenic speciation techniques

The present review describes the speciation techniques of arsenic. The principal advanced techniques discussed are gas chromatography, reversed-phase liquid chromatography, ion chromatography, capillary electrophoresis. Some other techniques are also mentioned. The extraction procedures of arsenic species from unknown samples are also discussed. Arsenic speciation is summarized in tabular form and optimizing parameters are also discussed.     

Roles of dynamic metal speciation and membrane permeability in metal flux through lipophilic membranes: general theory and experimental validation with nonlabile complexes

The study of the role of dynamic metal speciation in lipophilic membrane permeability in aqueous solution requires accurate interpretation of experimental data. To meet this goal, a general theory is derived for describing 1:1 metal complex flux, under steady-state and ligand excess conditions, through a permeation liquid membrane (PLM). The theory is applicable to fluxes through any lipophilic membrane. From this theory, fluxes in the three rate-limiting conditions for metal transport are readily derived, corresponding, namely, to (i) diffusion in the source solution, (ii) diffusion in the membrane, and (iii) the chemical kinetics of formation/dissociation of the metal complex in the interfacial reaction layer. The theory enables discussion of the reaction layer concept in a more general frame and also provides unambiguous criteria for the definition of an inert metal complex. The theoretical flux equations for fully labile complexes were validated in a previous paper. The general theory for semi- or nonlabile complexes is validated here by studying the flux of Pb(II) through PLMs in contact with solutions of Pb(II)-NTA and Pb(II)-TMDTA at different pHs and flow rates.     

Coupling techniques in speciation analysis

Summary Direct coupling techniques in speciation analysis lead to reproducible results with lower risks of trace contamination or losses of analytes in a shorter period of time. The direct coupling mode seems to be most promising for liquid chromatographic separation methods — especially gel permeation chromatography — and for some electrophoretic methods — as flow-through and capillary techniques. Promising detection methods are Flame-AAS and ICP-OES but also hydride-generation AAS and chemical reaction detectors in continuous-flow technique.     

Size fractionation techniques combined with INAA for speciation purposes

In natural waters trace elements, especially trace metals may be present in a variety of physicochemical forms. They may be associated with forms ranging from simple ions and molecules via hydrolysis products and colloids, pseudocolloids and organic or inorganic particles. The transition between categories is gradual. The presence of species differing in size, charge and density will influence on the transport, mobility and bioavailability of the trace element in question. Fractionation techniques which do not influence the distribution patterns are therefore required for speciation purposes. In the present work dialysis in situ and large membrane (hollow fibers) ultrafiltration are used for fractionation of low molecular weight species, colloids, pseudocolloids and particles. Due to the presence of foreign components transformation processes influence the distribution patterns of trace elements of interest. Sorption to foreign surfaces, complexation with agents present and aggregation of colloids (e.g., increasing ionic strength) result in a shift towards higher dimensions while desorption and dispersion processes mobilize the trace elements. Information on several components is therefore needed in speciation studies and a multielemental method of analysis having low determination limits must be applied. Instrumental neutron activation is appropriate to this kind of study because of its high sensitivity for simultaneous determination of a great-number of elements. Size fractionation techniques combined with INAA for the characterization of trace element species in natural waters will be discussed.     

Living organisms as an alternative to hyphenated techniques for metal speciation. Evaluation of baker's yeast immobilized on silica gel for Hg speciation

The use of living organisms for metal preconcentration and speciation is discussed. Among substrates, Saccharomyces cerevisiae baker's yeast has been successfully used for the speciation of mercury [Hg(II) and CH₃Hg⁺], selenium [Se(IV) and Se(VI)] and antimony [Sb(III) and Sb(V)]. To illustrate the capabilities of these organisms, the analytical performance of baker's yeast immobilized on silica gel for on-line preconcentration and speciation of Hg(II) and methylmercury is reported. The immobilized cells were packed in a PTFE microcolumn, through which mixtures of organic and inorganic mercury solutions were passed. Retention of inorganic and organic mercury solutions took place simultaneously, with the former retained in the silica and the latter on the yeast. The efficiency uptake for both species was higher than 95% over a wide pH range. The speciation was carried out by selective and sequential elution with 0.02 mol L⁻¹ HCl for methylmercury and 0.8 mol L⁻¹ CN⁻ for Hg(II). This method allows both preconcentration and speciation of mercury. The preconcentration factors were around 15 and 100 for methylmercury and mercury(II), respectively. The method has been successfully applied to spiked sea water samples.     

Biochemical speciation analysis by hyphenated techniques

The elucidation of mechanisms that govern the essentiality and toxicity of trace elements in living organisms is critically dependent upon the possibility of the identification, characterization and determination of chemical forms of these elements involved in life processes. The recent progress and the state-of-the-art of biochemical species-selective trace element analysis are critically evaluated with particular emphasis on the use of techniques combining the high selectivity of high performance liquid chromatography (HPLC) with the elemental or molecular specificity of mass spectrometry [using inductively coupled plasma (ICP) or electrospray ionization (ESI)]. The potential and limitations of hyphenated techniques as a tool for speciation of metals and metalloids in biological materials is discussed using a number of examples drawn from the latest research in the authors’ laboratory.     

Theoretical aspects of systems with competing interactions

This contribution from a non-theorist aims to give some introduction to the following theoretical papers as well as some of the points brought out in the oral discussion.     

Hydride-trapping techniques for the speciation of arsenic

The determination of arsenic species by the trapping of volatile hydrides prior to atomization in the light path of an atomic absorption spectrometer is described and its operation in the measurement of arsenic species in the marine environment are discussed. Examples are drawn from the analysis of Tamar estuary water and sediment interstitial (pore) waters and from studies of the temporal variation of dimethylarsenic in coastal waters. Improvements in both the design and operation of the technique have resulted in enhanced performance. Baseline resolution of inorganic arsenic, monomethylarsenic and dimethylarsenic is now possible and trimethylarsine is resolved. Ultraviolet photolysis of arsenobetaine and arsenocholine gives partial conversion to trimethylarsine oxide. This can be employed in the qualitative appraisal of the presence of trimethylarsenic species. Current detection limits (3 sigma) for inorganic, mono- and di-methylarsenic lie in the range 19 to 61 pg absolute, giving 19–61 ng/1 concentration detection limits for 1 ml samples. This can be improved even further by using larger sample volumes. The properties of the analysis system when presented with various arsenic species are described. A ca. 10% loss of arsenite occurs in samples stored at —20 °C and immediate freezing of samples in liquid nitrogen is recommended.     

Aluminium speciation: Clinical and environmental aspects

The speciation of aluminium in serum and in water is discussed. Ultramicrofiltration and HPLC techniques have been used to show that > 90% of the aluminium in serum is non-ultrafiltrable and bound to the protein transferrin. Regarding the toxicity and speciation of aluminium in natural waters the literature is plagued by disparities. The present lack of designed protocols and reference waters for the determination of the toxic aluminium species is thought to play a key role in producing these disagreements.